Study on Hot Cracking Susceptibility in Multipass Weld Metal of Ni-Base Super-Alloys

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This paper was carried out in order to make clear the cracking mechanism of microfissure in multipass weld metal of Ni-base super-alloys. Metallographic and fractographic examinations were performed to the fracture surface of microfissure. Moreover, the effects of P, Si and S on hot cracking susceptibilities in reheated weld metal of 70%Ni-Cr-Fe ternary alloys were evaluated in order to make clear the effects of these impurity elements on hot cracking susceptibilities in multipass weld metal of Ni-base welding rods. Main results obtained are as follows.(1) A detail fractographic examination has made clear that microfissure in multipass weld metal was liquation crack or ductility-dip crack occurring at reheated weld metal.(2) The liquation cracking susceptibilities in reheated weld metal of 70%Ni-Cr-Fe ternary alloys were too low and they increased with an increase in impurity elements. Ductility-dip cracking susceptibilities of materials containing S were much higher than those containing P and Si. Especially cracking susceptibilities of 0%Cr and 20%Cr alloys containing 0.007%S were higher than those of 5%Cr and 10%Cr alloys.(3) There was the suitable Cr content for reducing the ductility-dip cracking susceptibility because of contrary effects due to grain boundary carbide, hardness and grain boundary segregation of S.